Upgrade LM318 to OPA627 in Musical Fidelity P170?

Upgrading LM318 to OPA627 in Musical Fidelity P170 may have problem. Below is an extract of a thread on a DYI forum regarding swapping LM318 for OPA627 – there are a few issues with dropping them straight in

“They’re LM318’s – it was suggested some improvement might be had by swappingthem for 627’s, which I have coming with the caps, as I’m sure they will getused elsewhere if not on the P170… I think I’ve been infected with DIY..

I’ll have a go at creating a full schematic of the amp, and I’ll upload ithere and send it to Mr Hennesy, maybe he’ll archive it on his site…

Maybe on the weekend…

Michael

A 8Hennesy should have the schematics, I think I sent it to him some time ago.Regarding swapping the 318, I seem to remeber that there are no dc ofsetadjustments only a freq-compensatio pins/curcuits/components that you needto disconnect. Other then that it should work as is. The 637 will probablymake the amp go into oscillation.

maf_auOk, I have the schematics, thank you Mark and A 8 🙂

Here is what I see on pins 1 and 5 on the 318:

Pin1 via 3K3 and 2n2 to Pin5

So should I just hook them out and swap in the 627?

Regarding replacing the caps, especially the 22,000uf jobs on the PS:

How should I (safely) discharge them before working on them?

Is running the amp disconnected from the power for a while enough todischarge them, or should I ground the + terminal as well? Not wanting toget fried this week, and aware of the potential for stored current in thesepuppies…

At this stage, I’m going to replace caps, repair board and re-assemble. Ifeverything behaves at that point, I’ll swap the opamps.

Michael

amplifierguruFirstly, if you have any power resistors around say 5W’ers then work outfrom the supply voltage stored on the caps whether they would be in theirratings if placed across one C to ground i.e. P=VxV/R. Then just hold it on-the the other(s).

The RC on pins 1,5 sound like comp for the 318. I haven’t used one fordecades. Can you post the amp schema..

Cheers,Greg

maf_auquote:Originally posted by amplifierguruFirstly, if you have any power resistors around say 5W’ers then work outfrom the supply voltage stored on the caps whether they would be in theirratings if placed across one C to ground i.e. P=VxV/R. Then just hold it on-the the other(s).

The RC on pins 1,5 sound like comp for the 318. I haven’t used one fordecades. Can you post the amp schema..

Cheers,Greg

Thanks for the reply!

Ok, bear with me, I’m new to this. I’ve got the bit about grounding the capto earth via a power resistor, but calculating the load:

P=VxV/R

Am I on the right track here:

I’m thinking that V= the tranny output volts, R= the power resistor valueohms, so assuming I had a 5k/5w resistor, and the tranny was putting out63v:

A 8They are for comp so yes desolder them, there should also be a 5p Micabetween output and pin 8, disconnect pin 8 and do the swap. You may want tokeep it but connect the other end to the inverted opamp input. Cant rememberwhat I did.

I think I also added filmcaps to the opamps powersupply.

A 8I just sent you a private message.

BTW my unit had the K1058/J162 mosfets in them and it worked great with the627, your unit seems to have a lot of components replaced whichtheoretically can change how it behaves with new opamps. Be watchful of dclevel on the output of the amp and current drain when you first power it upafter placing the 627 in there.

amplifierguruYes maf_au you’re on track. generally when I design an amplifier I includedrain down resistors for the power supply. They only need bleed 5 mA or so.. so that by the time the lid is unscrewed they’re as good as discharged.

A8 – you designed this unit? So the pins 1,5 and 8 can be relieved ofcomponents to implant a OPA627?

Sorry I don’t know the design so without a schema.. I’m working blind.

Cheers,greg

maf_auOk, here it is.

Not sure what the limits are here, hopefully this is small enough…

Michael

maf_auquote:Originally posted by amplifierguruYes maf_au you’re on track. generally when I design an amplifier I includedrain down resistors for the power supply. They only need bleed 5 mA or so.. so that by the time the lid is unscrewed they’re as good as discharged.

A8 – you designed this unit? So the pins 1,5 and 8 can be relieved ofcomponents to implant a OPA627?

Sorry I don’t know the design so without a schema.. I’m working blind.

Cheers,greg

Thanks.

Looks like the replacement caps aren’t going to make it here today, I mightspend some time on the weekend identifying and recording the differencesbetween my amp and the schema, just in case there is something thatshouldn’t be…

Michael

amplifierguruYou’re doing well. If you can permanently fit 22k 1W resistors across allyour PS C’s that would ensure some safety. They won’t degrade performance.

The components on pins 1,5,8 can safely be removed if you want to use aOPA627. The 3K3 at the input filter may be picking up excess hum and noise,depending on the layout. Replacing with 1K or a parallel equiv might beworthwhile.

I wouldn’t use an OPA637 in this cct.”

By |
2009-10-10T13:45:00+00:00 October 10th, 2009|DIY_Audio_References|Comments Off on Upgrade LM318 to OPA627 in Musical Fidelity P170?